Remove warning when associated display is ADISPLAY_ID_NONE.

This warning caused logspam in emulator system image because emulator
host pre-registeris several touch screen devices which are not associated
with any display until multi-display feature is turned on. Unfortunately,
there is no easy way to add/remove touch screen device on-demand in
emulator host at runtime.

Bug: 160609749

Signed-off-by: Weilun Du <wdu@google.com>
Change-Id: I78d8c1b18285a58887b107d5469f9d1703132b44